The present invention concerns the field of digital radio communications with spread spectrum. It finds its application mainly in cellular networks using code division multiple access (CDMA) methods, for example in third generation networks of the universal mobile telecommunication system (UMTS) type.
The particular feature of spread spectrum techniques is to enable account to be taken of multiple propagation paths between the transmitter and the receiver, which generates an appreciable gain in reception diversity.
A receiver conventionally used for this is the rake receiver which comprises a certain number of “fingers” operating in parallel to estimate the digital symbols transmitted. The gain in reception diversity results from combining the estimates obtained in the different fingers of the receiver.
In a spread spectrum CDMA system, the symbols transmitted, usually binary (±1) or quaternary (±1±j), are multiplied by spreading codes composed of samples, called “chips”, the rate of which is greater than that of the symbols, in a ratio called the spread factor. Orthogonal or quasi-orthogonal spreading codes are allocated to different channels sharing the same carrier frequency in order to allow each receiver to detect the sequence of symbols intended for it, by multiplying the received signal by the corresponding spreading code.
The conventional rake receiver carries out a coherent demodulation based on an approximation of the impulse response of the radio propagation channel by a series of peaks, each peak appearing with a delay corresponding to the propagation time throughout the length of a particular path and having a complex amplitude corresponding to the attenuation and to the signal phase shift throughout the length of that path (instantaneous realization of fading). By analyzing several reception paths, that is by sampling on several occasions the output of a filter matched to the channel's spreading code, with delays corresponding respectively to those paths, the rake receiver obtains multiple estimates of the transmitted symbols, which are combined to obtain a gain in diversity. The combination can mainly be effected according to the method known as “maximum ratio combining” (MRC) which weights the different estimates according to the complex amplitudes observed for the different paths. To enable this coherent demodulation, pilot symbols can be transmitted with the information symbols for the estimate of the impulse response in the form of a succession of peaks.
Usually, in cellular systems, the fixed transceiver serving a given cell also transmits a marker signal on a pilot channel to which is allocated a determined pilot spreading code. This pilot code is communicated to the mobile terminals located in the cell or nearby, by means of system information transmitted by the base stations. The terminals take measurements of the power received on the pertinent pilot codes. These measurements enable mobiles on standby to identify the best cell to use if they have to make a random access. They also are used to identify, during a communication, the cell or cells with which the radio link conditions are the best for making an intercell communication transfer (“handover”) if necessary.
Another particular feature of spread spectrum CDMA systems is the ability to support a macrodiversity mode. Macrodiversity consists in envisaging that a mobile terminal can simultaneously communicate with distinct fixed transceivers of an active set. In the downlink direction, the mobile terminal receives the same information several times. In the uplink direction, the radio signal transmitted by the mobile terminal is captured by the fixed transceivers of the active set to form different estimates subsequently combined in the network.
Macrodiversity procures a reception gain which improves the performance of the system by combining different observations of the same information.
It is also used to perform soft handovers (SHO) when the mobile terminal moves.
The macrodiversity mode leads, in the rake receiver of the mobile terminal, to assigning the fingers allocated to a communication to paths belonging to different propagation channels from several fixed transceivers and usually having different spreading codes.
On the network side, the macrodiversity mode implements a kind of macroscopic rake receiver, the fingers of which are located in different transceivers. The estimates are combined after channel decoding in a base station if the base station groups together all the transceivers concerned, or if not, in a controller supervising the base stations.
The macrodiversity mode imposes a certain signaling load in the network when the active set relating to a terminal must be updated. Furthermore, it mobilizes supplementary transmission and reception resources in the base stations, as well as some bandwidth for the transfer of the data to be combined in the network. It is therefore judicious to use it only when the reception gain obtained is significant.
This reception gain comes principally from the multiplicity of propagation paths taken into consideration. There are many cases in which a propagation channel (or a small number of such channels) have sufficiently numerous paths that the addition of one or more supplementary transceivers to the active set procures only a weak gain in terms of bit error ratio (BER), even though the reception conditions are correct on the propagation channels between the terminal and these supplementary transceivers. In such a case, the macrodiversity links load the network to no great purpose.
In a CDMA system such as the UMTS, the transmit power over the radio interface is adjusted through a feedback control procedure in which the receiver returns power control commands (TPC) to the transmitter to try to achieve an objective in terms of reception conditions. These TPC commands consist of bits transmitted at a fairly high rate and their value indicates whether the transmit power should be increased or reduced.
In the case of a communication in macrodiversity, the different fixed transceivers of the active set receive identical TPC bits from the mobile terminal. Respective corrective terms can be taken into account by these fixed transceivers to balance the transmitted powers. For a given active set, if a first transceiver generates a large number of propagation paths whereas a second generates only a small number of paths, it may be preferable to aim for a higher power set-point value for the first transceiver than for the second. Otherwise, it may happen that the gain in macrodiversity brought about by adding the second transceiver to the active set is negative.
Since the chip rate is fixed, a high rate physical channel has a low spread factor and a short symbol duration. If the impulse response of this channel comprises paths that are relatively widely spaced over time, the result is inter-symbol interference which degrades the performance of the receiver or requires a channel equalizer which greatly increases its complexity. It may therefore be advantageous to divide such a channel into two channels of double the spread factor. However, the multiplication of channels with high spread factors is not always desirable, so it is better to dispense therewith when the channel generates almost no inter-symbol interference.
An object of the present invention is to optimize the use of the resources in a radio network with spread spectrum.